In software design, we can say a class is a polymorphic class. If a class is not a polymorphic class and I want to make it one, do I say I've polymorphized the class?
Polymorph is a word software folks created from the some handy Greek-ish roots poly (meaning "many") and morph (meaning "change"). The idea is to indicate that the base type (aka: class) in question can be easily changed in many possible ways (generally via a derived class of some kind). It is generally used as an adjective (it modifies a noun) or an adverb (modifying a verb like is).
So back to English grammar. Often we can transform a noun to a verb with an "ize" on the end to indicate that we have somehow transformed the subject of the sentence into that other noun. For example, if I transform an array into a raster, you could say I rasterized the array.
However, its kind of awkward to do this with an adjective or adverb rather than a noun. I'm sure its done from time to time, but not ubiquitously. For example, if I paint something blue, you wouldn't likely hear someone say that I blueized that thing.
Now you could try to transform your noun into a verb just by adding a simple -ed on the end (polymorphed). However, as I think you suspected, that would be saying that you actually changed the subject itself, which isn't the point of what you want to say at all.
So I don't think you can get around the somewhat wordy, "made it polymorphic".
You can if you want to.
It’s not in common usage yet. A phrasing like “…made
someclass polymorphic…” is currently more common (as googling, and more specifically searching various programming language communities, confirms). However, in technical contexts, coining new terms is quite common, and not generally scoffed at; and this one is completely transparent — anyone familiar with polymorphism will understand what you mean. Also, while uncommon, it’s certainly not unknown: searching does give a fair few other examples of people using polymorphize (or polymorphise) as a verb.
So: if you like it, go for it!